1. Technical Field
The present invention relates to marking articles or other workpieces and, more particularly, to methods of laser marking or laser imprinting a workpiece, and to the resultant article produced thereby and to compositions used therein.
2. Description of the Prior Art
Transferring text and graphics to metal, plastic, wood or composite pieces is commonly done by mechanical engraving (followed by a paint fill) or surface painting.
Mechanical engraving into, for example, a metal article followed by paint fill is often a tedious multi-step process. Typically, the article (i) is mechanically engraved using a hard tool engraving system, (ii) is deburred, cleaned, and dried, (iii) is filled with a solvent-based or a powdered paint, (iv) is cured in an oven and, (v) then is cleaned to remove excess paint deposited outside of engraved areas.
A majority of mechanical engraving systems are limited to engraving flat surfaces. Furthermore, strength limitations on mechanical cutters limit their minimum diameter and, hence, their stroke width. In many cases, metals, particularly high nickel-chromium alloy steels, require slow processing speeds with the assistance of cutting fluids. The use of these fluids requires a cleanup operation prior to painting. The paint filling process can be quite difficult, particularly with intricate graphics. Placement of the paint at this stage is very important because misplacement or overfilling will require a substantial cleanup effort after the paint has been oven cured. Some metals, such as a mirror finish stainless steel or brass, scratch quite easily; thus, special care must be taken not to damage these surfaces during paint filling and cleanup operations.
Although time consuming, mechanically engraved text and graphics have durability and permanence. When filled with colored paint, the final product is long lasting and aesthetically pleasing.
Surface painting involves transferring text and graphics onto workpiece surfaces using silk-screen or ink jet techniques. Surface painting can be done fairly quickly and in a full range of colors.
One type of surface painting uses dry powder paint. Typically, powdered paint is given an electrical charge, which attracts the powdered paint to the surface of the article to be painted. The physical properties and chemical composition of the powder paint are specifically designed for the particular process used to fuse the paint to the surface. One such process involves entraining powdered polymer in an air stream and passing it through a corona discharge plasma. This process uses commercially available powder paint having relatively large particle sizes, on the order of 10-86 .mu.m. However, in many cases, the present inventors believe that a lack of durability or permanence required for long term wear, such as for elevator cab interior markings, constitutes a significant drawback to surface painting. This process is not commonly used to transfer text and graphics onto workpiece surfaces.
Another process, used in conventional copying machines, formulates the toner, e.g. a polymer powder, with various charge transfer agents. Small particle size (approximately 7-10 .mu.m) is required in this process to facilitate the flow of powder paint from a hopper, to a charged developer roller, then to a second drum coated with photoreceptor, and lastly onto the paper. Fusion of the toner to paper is accomplished using a hot roller. An alternative method to fix toner to paper is known as flash fusion and uses xenon lamps to heat and fuse the toner onto the paper. The lamps produce a strong emission intensity from approximately 800-1100 nm, which readily fuses black toners since they have strong absorption in this near infrared region. For color toners, however, an absorption additive, such as diammonium salt that absorbs in the 800-1100 nm region, is used to facilitate fusion.
More recently, laser engraving and laser marking techniques have been developed as alternatives to both mechanical engraving and surface painting of materials, such as metals, plastics, composites or wood. Conventional laser engraving systems utilize, for example, either a Nd-YAG or a CO.sub.2 laser. A galvanometer driven motor arrangement or an X-Y gantry arrangement steers a focused laser beam over a workpiece surface.
Laser engraving systems work well as versatile engraving tools, particularly for intricate engraving because of their fine stroke width (0.003"-0.010"), and for engraving into moderately contoured surfaces. Because laser engraving is a non-contact process (method), problems associated with mechanical engraving such as tool wear and resharpening, tool breakage, cutting coolants and part fixturing are eliminated. Furthermore, relatively rapid part engraving is possible with known laser engraving systems. However, even with the known laser engraving systems, there is a significant limitation to engraving depth into dense materials such as stainless steel or brass. Edge definition is degraded increasingly with depth because of a build-up of, for example, remelted metal. A minimum depth of approximately (.+-.10%) 0.010" is required for subsequent paint filling. This requires having to deburr and to clean the engraved areas prior to any paint filling. Even more difficult than for conventional mechanical engraving is the paint filling operation after laser engraving, which operation includes an application of paint, a curing of the paint, and a removal of excess paint, all without damaging the finished surfaces. Powdered paint, as opposed to a liquid paint, is easier to employ, but typically, the overall paint filling operation is an intricate, multi-step process.
As an alternative to surface painting or to engraving (mechanical or laser) combined with paint filling, there is laser marking. Such marking is dependent upon a contrast produced at a point at which the laser beam strikes or impinges upon the material surface. To achieve a good contrast, the laser power can be optimized to produce dark brown or black oxides on shiny metal surfaces or to remove colored oxides or paint from pretreated panels to expose a lighter contrasting substrate. For many applications, laser marking works well. However, as with silk screening or ink jet printing techniques, the limited durability or limited permanence of the mark effectively excludes use of known laser marking techniques for applications such as use for elevator cab interior markings, because those markings are often subjected to extensive abrasive contact with, e.g., elevator passengers' hands. Furthermore, the lack of contrast which results when obliquely viewing surface markings on shiny metals seems to the present inventors to pose an additional shortcoming of known laser marking techniques for elevator system applications. For a discussion of some known laser marking systems and techniques, see, for example, Industrial Strength Laser Marking: Turning Photons into Dollars, published by Control Laser Corporation, Orlando, Fla. (1992), which publication is hereby incorporated in its entirety by reference.